API examples
This documentation provides simple examples on how to use the tesseract-ocr API (v3.02.02-4.0.0) in C++. It is expected that tesseract-ocr is correctly installed including all dependencies. It is expected the user is familiar with C++, compiling and linking program on their platform, though basic compilation examples are included for beginners with Linux.
More details about tesseract-ocr API can be found at baseapi.h.
Basic example
Code:
#include <tesseract/baseapi.h>
#include <leptonica/allheaders.h>
int main()
{
char *outText;
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
// Initialize tesseract-ocr with English, without specifying tessdata path
if (api->Init(NULL, "eng")) {
fprintf(stderr, "Could not initialize tesseract.\n");
exit(1);
}
// Open input image with leptonica library
Pix *image = pixRead("/usr/src/tesseract/testing/phototest.tif");
api->SetImage(image);
// Get OCR result
outText = api->GetUTF8Text();
printf("OCR output:\n%s", outText);
// Destroy used object and release memory
api->End();
delete api;
delete [] outText;
pixDestroy(&image);
return 0;
}
The program must be linked to the tesseract-ocr and leptonica libraries.
If you want to restrict recognition to a sub-rectangle of the image - call SetRectangle(left, top, width, height) after SetImage. Each SetRectangle clears the recognition results so multiple rectangles can be recognized with the same image. E.g.
api->SetRectangle(30, 86, 590, 100);
GetComponentImages example
Pix *image = pixRead("/usr/src/tesseract/testing/phototest.tif");
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
api->Init(NULL, "eng");
api->SetImage(image);
Boxa* boxes = api->GetComponentImages(tesseract::RIL_TEXTLINE, true, NULL, NULL);
printf("Found %d textline image components.\n", boxes->n);
for (int i = 0; i < boxes->n; i++) {
BOX* box = boxaGetBox(boxes, i, L_CLONE);
api->SetRectangle(box->x, box->y, box->w, box->h);
char* ocrResult = api->GetUTF8Text();
int conf = api->MeanTextConf();
fprintf(stdout, "Box[%d]: x=%d, y=%d, w=%d, h=%d, confidence: %d, text: %s",
i, box->x, box->y, box->w, box->h, conf, ocrResult);
boxDestroy(&box);
}
delete api;
Result iterator example
It is possible to get confidence value and BoundingBox per word from a ResultIterator:
Pix *image = pixRead("/usr/src/tesseract/testing/phototest.tif");
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
api->Init(NULL, "eng");
api->SetImage(image);
api->Recognize(0);
tesseract::ResultIterator* ri = api->GetIterator();
tesseract::PageIteratorLevel level = tesseract::RIL_WORD;
if (ri != 0) {
do {
const char* word = ri->GetUTF8Text(level);
float conf = ri->Confidence(level);
int x1, y1, x2, y2;
ri->BoundingBox(level, &x1, &y1, &x2, &y2);
printf("word: '%s'; \tconf: %.2f; BoundingBox: %d,%d,%d,%d;\n",
word, conf, x1, y1, x2, y2);
delete[] word;
} while (ri->Next(level));
}
delete api;
It is also possible to use other iterator levels (block, line, word, etc.), see PageiteratorLevel.
Orientation and script detection (OSD) example
const char* inputfile = "/usr/src/tesseract/testing/eurotext.tif";
tesseract::Orientation orientation;
tesseract::WritingDirection direction;
tesseract::TextlineOrder order;
float deskew_angle;
PIX *image = pixRead(inputfile);
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
api->Init("/usr/src/tesseract/", "eng");
api->SetPageSegMode(tesseract::PSM_AUTO_OSD);
api->SetImage(image);
api->Recognize(0);
tesseract::PageIterator* it = api->AnalyseLayout();
it->Orientation(&orientation, &direction, &order, &deskew_angle);
printf("Orientation: %d;\nWritingDirection: %d\nTextlineOrder: %d\n" \
"Deskew angle: %.4f\n",
orientation, direction, order, deskew_angle);
delete api;
Explanation for result codes are in publictypes.h
Example of iterator over the classifier choices for a single symbol
Pix *image = pixRead("/usr/src/tesseract/testing/phototest.tif");
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
api->Init(NULL, "eng");
api->SetImage(image);
api->SetVariable("save_blob_choices", "T");
api->SetRectangle(37, 228, 548, 31);
api->Recognize(NULL);
tesseract::ResultIterator* ri = api->GetIterator();
tesseract::PageIteratorLevel level = tesseract::RIL_SYMBOL;
if(ri != 0) {
do {
const char* symbol = ri->GetUTF8Text(level);
float conf = ri->Confidence(level);
if(symbol != 0) {
printf("symbol %s, conf: %f", symbol, conf);
bool indent = false;
tesseract::ChoiceIterator ci(*ri);
do {
if (indent) printf("\t\t ");
printf("\t- ");
const char* choice = ci.GetUTF8Text();
printf("%s conf: %f\n", choice, ci.Confidence());
indent = true;
} while(ci.Next());
}
printf("---------------------------------------------\n");
delete[] symbol;
} while((ri->Next(level)));
}
delete api;
Example to get confidence for alternative symbol choices per character for LSTM
#include <tesseract/baseapi.h>
#include <leptonica/allheaders.h>
int main()
{
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
// Initialize tesseract-ocr with English, without specifying tessdata path
if (api->Init(NULL, "eng")) {
fprintf(stderr, "Could not initialize tesseract.\n");
exit(1);
}
// Open input image with leptonica library
Pix *image = pixRead("/home/ubuntu/tesseract/test/testing/trainingital.tif");
api->SetImage(image);
// Set lstm_choice_mode to alternative symbol choices per character, bbox is at word level.
api->SetVariable("lstm_choice_mode", "2");
api->Recognize(0);
tesseract::PageIteratorLevel level = tesseract::RIL_WORD;
tesseract::ResultIterator* res_it = api->GetIterator();
// Get confidence level for alternative symbol choices. Code is based on
// https://github.com/tesseract-ocr/tesseract/blob/a7a729f6c315e751764b72ea945da961638effc5/src/api/hocrrenderer.cpp#L325-L344
std::vector<std::vector<std::pair<const char*, float>>>* choiceMap = nullptr;
if (res_it != 0) {
do {
const char* word;
float conf;
int x1, y1, x2, y2, tcnt = 1, gcnt = 1, wcnt = 0;
res_it->BoundingBox(level, &x1, &y1, &x2, &y2);
choiceMap = res_it->GetBestLSTMSymbolChoices();
for (auto timestep : *choiceMap) {
if (timestep.size() > 0) {
for (auto & j : timestep) {
conf = int(j.second * 100);
word = j.first;
printf("%d symbol: '%s'; \tconf: %.2f; BoundingBox: %d,%d,%d,%d;\n",
wcnt, word, conf, x1, y1, x2, y2);
gcnt++;
}
tcnt++;
}
wcnt++;
printf("\n");
}
} while (res_it->Next(level));
}
// Destroy used object and release memory
api->End();
delete api;
pixDestroy(&image);
return 0;
}
Example to get HOCR output with alternative symbol choices per character (LSTM)
This is similar to running tesseract from commandline with -c lstm_choice_mode=2 hocr.
#include <tesseract/baseapi.h>
#include <leptonica/allheaders.h>
int main()
{
char *outText;
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
// Initialize tesseract-ocr with English, without specifying tessdata path
if (api->Init(NULL, "eng")) {
fprintf(stderr, "Could not initialize tesseract.\n");
exit(1);
}
// Open input image with leptonica library
Pix *image = pixRead("/tesseract/test/testing/trainingital.tif");
api->SetImage(image);
api->SetVariable("lstm_choice_mode", "2");
// Get HOCR result
outText = api->GetHOCRText(0);
printf("HOCR alternative symbol choices per character :\n%s", outText);
// Destroy used object and release memory
api->End();
delete api;
delete [] outText;
pixDestroy(&image);
return 0;
}
Notice the different confidence values for:
<span class='ocrx_cinfo' id='lstm_choices_1_4_4'>
<span class='ocr_glyph' id='choice_1_4_7' title='x_confs 83'>m</span>
<span class='ocr_glyph' id='choice_1_4_8' title='x_confs 16'>n</span>
</span>
<span class='ocrx_cinfo' id='lstm_choices_1_4_5'>
<span class='ocr_glyph' id='choice_1_4_9' title='x_confs 54'>n</span>
<span class='ocr_glyph' id='choice_1_4_10' title='x_confs 17'>i</span>
<span class='ocr_glyph' id='choice_1_4_11' title='x_confs 15'>m</span>
<span class='ocr_glyph' id='choice_1_4_12' title='x_confs 6'>a</span>
<span class='ocr_glyph' id='choice_1_4_13' title='x_confs 4'>u</span>
<span class='ocr_glyph' id='choice_1_4_14' title='x_confs 1'>r</span>
<span class='ocr_glyph' id='choice_1_4_15' title='x_confs 1'>e</span>
</span>
Compiling C++ API programs on Linux
Including and linking to Tesseract’s API is done in a standard Linux way. To compile a basic program against the API, you can use a command like this:
g++ -o myprogram myprogram.cpp -llept -ltesseract
If Tesseract is installed in an unusual place, you can specify the include and lib directories explicitly with g++’s -I and -L flags, like this:
g++ -o myprogram myprogram.cpp -I/home/nick/local/include/tesseract -L/home/nick/local/lib -llept -ltesseract
C-API in python
Tesseract-ocr from version 3.02.02 provide C-API. This enable to use tesseract-ocr shared library in python (and other languages that can use C libraries):
import os
import ctypes
lang = "eng"
filename = "/usr/src/tesseract-ocr/phototest.tif"
libname = "/usr/local/lib64/libtesseract.so.3"
TESSDATA_PREFIX = os.environ.get('TESSDATA_PREFIX')
if not TESSDATA_PREFIX:
TESSDATA_PREFIX = "../"
tesseract = ctypes.cdll.LoadLibrary(libname)
tesseract.TessVersion.restype = ctypes.c_char_p
tesseract_version = tesseract.TessVersion()
api = tesseract.TessBaseAPICreate()
rc = tesseract.TessBaseAPIInit3(api, TESSDATA_PREFIX, lang)
if (rc):
tesseract.TessBaseAPIDelete(api)
print("Could not initialize tesseract.\n")
exit(3)
text_out = tesseract.TessBaseAPIProcessPages(api, filename, None, 0)
result_text = ctypes.string_at(text_out)
print 'Tesseract-ocr version', tesseract_version
print result_text
Example of passing python file object to C-API can be found at pastebin.
Example of extracting orientation from Tesseract 4.0:
# /usr/bin/env python3
# coding: utf-8
PATH_TO_LIBTESS = '/path/to/development/libtesseract.so'
import cffi # requires "pip install cffi"
ffi = cffi.FFI()
ffi.cdef("""
struct Pix;
typedef struct Pix PIX;
PIX * pixRead ( const char *filename );
char * getLeptonicaVersion ( );
typedef struct TessBaseAPI TessBaseAPI;
typedef int BOOL;
const char* TessVersion();
TessBaseAPI* TessBaseAPICreate();
int TessBaseAPIInit3(TessBaseAPI* handle, const char* datapath, const char* language);
void TessBaseAPISetImage2(TessBaseAPI* handle, struct Pix* pix);
BOOL TessBaseAPIDetectOrientationScript(TessBaseAPI* handle, char** best_script_name,
int* best_orientation_deg, float* script_confidence,
float* orientation_confidence);
""")
libtess = ffi.dlopen(PATH_TO_LIBTESS)
from ctypes.util import find_library
liblept = ffi.dlopen(find_library('lept'))
ffi.string(libtess.TessVersion())
ffi.string(liblept.getLeptonicaVersion())
api = libtess.TessBaseAPICreate()
libtess.TessBaseAPIInit3(api, ffi.NULL, ffi.NULL)
pix = liblept.pixRead('mono.png'.encode())
libtess.TessBaseAPISetImage2(api, pix)
script_name = ffi.new('char **')
orient_deg = ffi.new('int *')
script_conf = ffi.new('float *')
orient_conf = ffi.new('float *')
libtess.TessBaseAPIDetectOrientationScript(api, script_name, orient_deg, script_conf, orient_conf)
print(ffi.string(script_name).decode('utf-8'))
print(orient_deg[0])
print(script_conf[0])
print(orient_conf[0])
Example using the C-API in a C program
The C-API can of course also be used by regular C programs, as in this very basic example.
#include <stdio.h>
#include <allheaders.h>
#include <capi.h>
void die(const char *errstr) {
fputs(errstr, stderr);
exit(1);
}
int main(int argc, char *argv[]) {
TessBaseAPI *handle;
PIX *img;
char *text;
if((img = pixRead("img.png")) == NULL)
die("Error reading image\n");
handle = TessBaseAPICreate();
if(TessBaseAPIInit3(handle, NULL, "eng") != 0)
die("Error initializing tesseract\n");
TessBaseAPISetImage2(handle, img);
if(TessBaseAPIRecognize(handle, NULL) != 0)
die("Error in Tesseract recognition\n");
if((text = TessBaseAPIGetUTF8Text(handle)) == NULL)
die("Error getting text\n");
fputs(text, stdout);
TessDeleteText(text);
TessBaseAPIEnd(handle);
TessBaseAPIDelete(handle);
pixDestroy(&img);
return 0;
}
On Linux you can compile it as you would build a program using the C++ API.
Example creating searchable pdf from image in C++
#include <leptonica/allheaders.h>
#include <tesseract/baseapi.h>
#include <tesseract/renderer.h>
int main()
{
const char* input_image = "/usr/src/tesseract-oc/testing/phototest.tif";
const char* output_base = "my_first_tesseract_pdf";
const char* datapath = "/Projects/OCR/tesseract/tessdata";
int timeout_ms = 5000;
const char* retry_config = nullptr;
bool textonly = false;
int jpg_quality = 92;
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
if (api->Init(datapath, "eng")) {
fprintf(stderr, "Could not initialize tesseract.\n");
exit(1);
}
tesseract::TessPDFRenderer *renderer = new tesseract::TessPDFRenderer(
output_base, api->GetDatapath(), textonly, jpg_quality);
bool succeed = api->ProcessPages(input_image, retry_config, timeout_ms, renderer);
if (!succeed) {
fprintf(stderr, "Error during processing.\n");
return EXIT_FAILURE;
}
api->End();
delete api;
return EXIT_SUCCESS;
}
Example of monitoring OCR progress in C++
#include <tesseract/baseapi.h>
#include <tesseract/ocrclass.h>
#include <leptonica/allheaders.h>
#include <thread>
void monitorProgress(ETEXT_DESC *monitor, int page);
void ocrProcess(tesseract::TessBaseAPI *api, ETEXT_DESC *monitor);
void monitorProgress(ETEXT_DESC *monitor, int page) {
while (1) {
printf( "\r%3d%%", monitor[page].progress);
fflush (stdout);
if (monitor[page].progress==100)
break;
}
}
void ocrProcess(tesseract::TessBaseAPI *api, ETEXT_DESC *monitor) {
api->Recognize(monitor);
}
int main() {
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
ETEXT_DESC *monitor = new ETEXT_DESC();
if (api->Init("/tesseract/tessdata_best", "eng")) {
fprintf(stderr, "Could not initialize tesseract.\n");
return 1;
}
api->SetPageSegMode(tesseract::PSM_AUTO);
Pix *image = pixRead("/tesseract-ocr/test/testing/phototest.tif");
if (!image) {
fprintf(stderr, "Leptonica can't process input file!\n");
return 2;
}
api->SetImage(image);
int page = 0;
std::thread t1(ocrProcess, api, monitor);
std::thread t2(monitorProgress, monitor, page);
t1.join();
t2.join();
pixDestroy(&image);
char *outText = api->GetUTF8Text();
printf("\n%s", outText);
if (outText)
delete [] outText;
api->End();
delete api;
return 0;
}
More complex examples (e.g. cancelling OCR process) can be found in source code of TesseractGui, gimagereader or android textfairy app.